1. Field of the Invention
The present invention relates to a nitride semiconductor laser diode, and more particularly, to a method for manufacturing a nitride semiconductor laser diode having a ridge structure on an n-type layer.
2. Discussion of the Related Art
Generally, a nitride semiconductor laser diode has been developed and manufactured to be applied to a mass information storage device and a color printer. Also, there have been various applications introduced by using the nitride semiconductor laser diode.
For applying the nitride semiconductor laser diode to the mass information storage device and the color printer, the nitride semiconductor laser diode is required to have characteristics including low threshold current (Ith), high external quantum efficiency (ηex) and low operating voltage (Vop) for providing high reliability related to a lifetime and low power consumption.
The low operating voltage is a major factor to be improved in the nitride semiconductor laser diode. That is, a current-voltage characteristic (I-V) of the nitride semiconductor laser diode must be improved.
A conventional nitride semiconductor laser diode will be described below with reference to FIG. 1.
FIG. 1 is a cross-sectional view of a conventional nitride semiconductor laser diode. As shown in FIG. 1, the conventional nitride semiconductor laser diode includes an undoped GaN layer 1 formed on a substrate (not shown) made of a GaN or a sapphire, an n-GaN layer 2, a compliance layer (InGaN) 3, an n-cladding layer (n-ALGaN) 4, an n-waveguide (GaN) layer 5, a multi-quantum well (MQW) 6, an electron blocking layer (EBL) 7, a p-waveguide layer (GaN) 8, a p-cladding layer (p-AlGaN) 9, a capping layer (p-GaN) 10, and a p-pad metal layer (not shown) formed on the capping layer 10.
In FIG. 1, the n-GaN layer 2 is extended to a right and a left directions with a substrate (not shown) and an exposed n-pad metal layer (not shown) is formed by mesa etching the expended the n-GaN layer 2 and the substrate (not shown).
The conventional nitride semiconductor laser diode is a p-n diode form. That is, the conventional nitride semiconductor laser diode includes the multi-quantum well MQW 6 emitting lights, and GaN waveguide layers 5 and 8 and AlGaN cladding layers 4, 9 formed by surrounding the multi-quantum well 6 as a center.
The nitride semiconductor laser diode receives current from the p-pad metal layer (not shown) and emits the lights by coupling an electron and a hole at the MQW 6. The light is externally radiated through the capping layer (p-GaN) 10.
When emitting the light, the GaN waveguide layers 5, 8 and the AlGaN Cladding layers 4, 9 concentrate the electron and the hole in the MQW 6 and guide the emitted lights.
For effectively emitting a laser beam and improving beam characteristics of a laser diode, a ridge structure has been implemented to a nitride semiconductor laser diode.
FIG. 2 is a cross-sectional view of a conventional nitride semiconductor laser diode having a ridge structure.
As shown in FIG. 2, the nitride semiconductor laser diode having the ridge structure includes: a p-cladding layer 9 formed on an MQW 6, a EBL 7 and a P-waveguide layer 8, where a center part of the p-cladding layer 9 is projected and the MQW 6, the EBL 7 and the P-waveguide layer 8 are identical to the same of FIG. 1; a capping layer 10 formed on an upper surface of the projected part of p-cladding layer 9; and an ohmic contact metal 11 formed on the capping layer 10. In FIG. 2, the projected part of the p-cladding layer 9, the capping layer 10 and the ohmic contact metal layer 11 are formed the ridge structure.
The ridge structure is generally manufactured to have a width less than 3 μm and a current from the p-pad metal layer 13 is flowed through the ridge structure.
An insulating layer 12 is formed at side surfaces of the ridge structure and on an upper surface of the p-waveguide layer 8. After forming the insulating layer 12, a p-pad metal layer 13 is formed on the insulating layer 12 and the ohmic contact metal layer 11.
According to the above-mentioned related art, the ridge structure is generally formed on the p-type layers 8, 9 and 10.
When the p-type layer is grown, a magnesium Mg is used as a dopant. The p-type layer must be formed after forming the n-type layer, the waveguide layer and the MQW because the Mg has a memory effect. Accordingly, it is structurally simple and convenient to form the ridge structure on the p-type layer.
However, the nitride has a large band gap, a low carrier concentration and a low mobility. Therefore, it is difficult to form the ohmic contact metal layer 11 comparing to other compound semiconductor.
Also, a resistance seriously increases since the p-type layer has low carrier concentration and low mobility comparing to the n-type layer and it is difficult to form the ohmic contact metal 11.
Furthermore, the ridge structure according to the related art causes sudden increase of element resistance. It degrades I-V characteristics of the nitride semiconductor laser diode.
The increase of resistance induces to increase an operating voltage and causes to generate heat from a part of the ridge contacted to the p-pad metal. Therefore, the p-ohmic metal and element characteristics are degraded and the reliability of the laser diode such as the lift time decreases.